Water and gases (particularly hydrogen) can cause substantial damage if they penetrate into a repeater housing. For instance electronic components, particularly semi-conductors, metal oxide resistors, and surface acoustic wave filters could be damaged by water or gas ingress. If water or gases have penetrated into a submarine cable, which can occur as a result of permeation or cable damage for example, then it is clearly important to prvent them from entering a repeater housing.
Optical fibres contained in a submarine cable are often provided with a protective sheath which may comprise a primary layer, of silicone rubber for instance, and a secondary layer of, say, a polymeric material. A potential path for water and gases into a repeater housing is provided by any optical fibre feed through into the housing, the path being along the layers of the protective sheath of the optical fibre.
In the event of certain types of damage, perhaps affecting the cable to repeater housing joint, then water or gas at considerable pressure may be present at the optical fibre feed through to the repeater housing. It is desirable that the feed through should not allow water or gas ingress even under these conditions.
A second factor affecting the design of a feed through is that optical fibres are extremely fragile. The protection afforded by a protective sheath is therefore a very important factor and preferably should be preserved in some manner even at an optical fibre feed through.